MIPS: Implement TARGET_INSN_COSTS

When combine some instructions, the generic `rtx_cost`
may over estimate the cost of result RTL, due to that
the RTL may be quite complex and `rtx_cost` has no
information that this RTL can be convert to simple
hardware instruction(s).

In this case, Let's use `insn_count * perf_ratio` to
estimate the cost if both of them are available.
Otherwise fallback to pattern_cost.

When non-speed, Let's use the length as cost.

gcc

	* config/mips/mips.cc (mips_insn_cost): New function.

gcc/testsuite

	* gcc.target/mips/data-sym-multi-pool.c: Skip Os or -O0.